A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Historically, in lithographic tools a mounting side and a patterned side of a reticle are one and the same, establishing a reticle focal plane at a plane of a reticle stage plate. Thus, knowledge of stage position in six degrees of freedom (DOF) results in knowledge of the reticle patterned surface position in six DOF. The six DOF are X, Y, Z, Rx, Ry, and Rz, as shown in FIG. 2. However, mounting (or clamping) of an extreme ultra violet (EUV) reticle will almost certainly be to a back surface of the reticle (e.g., opposite from the patterned surface). Backside clamping results in a reticle focal plane position relative to the reticle stage that is a function of reticle height and tilt profile. Thus, in contrast to deep ultra violet (DUV) systems, knowledge of the reticle stage position does not resolve where the pattern of the reticle is located in all six DOF. The out-of-plane DOF (Z, Rx, and Ry) cannot be easily determined due to the thickness variation of the reticle. The position of the patterned side (opposite to the clamped side) of the reticle needs to be known accurately in all six DOF. Hereinafter, the aforementioned patterned surface of the reticle is referred to in the text and claims as the reticle surface or patterned reticle surface.
In almost all steppers and scanners three in-plane DOF (X, Y, and Rz) are determined from typical stage metrology schemes using interferometers. However, three out-of-plane DOF (Z, Ry, and Rx) are more difficult to measure. As discussed above, in an EUV tool, Z, Rx, and Ry have to be known with much higher accuracy than in previous lithography tools. The accuracy requirement stems from the need to position the pattern on the reticle at a focal plane related to optics of the lithography tool. Also, in some cases, optics are not telecentric at the reticle focal plane, which increases the need for accurately determining the reticle position on the reticle stage to within six DOF. The focal place is also referred as the best object plane. At the same time, it is critical to accurately maintain focus on the pattern on the reticle even though the reticle has a certain height and tilt profile with respect to the reticle stage. Therefore, measuring the Z position and the out of plane tilts (Rx and Ry) of the patterned side of the reticle in the EUV tool requires tight accuracy.
An embodiment of a measuring system is known from U.S. Pat. No. 6,934,005. The known system comprises an interferometer arranged to provide input data related to a height and tilt profile of the reticle surface. These data are subsequently processed by a suitable computer program to linearly approximate the height and the tilt profile of the reticle surface. For this purpose the reticle comprises two reflective paths arranged substantially at opposite sides of the reticle, notably extending along the y-direction of a coordinate system assigned to the reticle and the reticle stage, see FIG. 2. The interferometer data is used to approximate the reticle surface, which is used for controlling a linear displacement of the reticle stage during a suitable exposure of the substrate.
It is disadvantage of the known method that considerable distortions occur when the reticle stage moves linearly during the exposure of the substrate.